The present invention relates to a base station using an adaptive array antenna and included in a radio communication system represented by a cellular radio communication system. The present invention is effective particularly in application to a system employing a code-division multiple access system (CDMA system) and controlling transmitting power for a downlink.
An adaptive array antenna (hereinafter abbreviated to xe2x80x9cAAAxe2x80x9d) has attracted a great deal of attention in recent years as mentioned in NTT DoCoMo Technical Journal, Vol. 5, No. 4, pp. 25-36 (June, 1998). When applying an AAA to radio communication represented by cellular communication, the AAA is installed, in most cases, at a base station because the AAA has large construction.
There have been made many studies of the AAA relating to uplinks. Since the AAA forms an appropriate beam and does not radiate radio waves in undesired directions, it is expected that the capacity of downlinks can greatly be increased by using the AAA. In a time-division duplex system (hereinafter referred to as xe2x80x9cTDD systemxe2x80x9d), an uplink and a downlink are used for the same frequency in a dime-division mode. Therefore, the timing of a pair of uplink and downlink is sufficiently small as compared with a fading frequency, the used propagation paths can be regarded as the same paths and transmission can be achieved by using the weight of the same array antenna as that for the uplink.
In a frequency-division duplex system (hereinafter referred to as xe2x80x9cFDD systemxe2x80x9d), an uplink and a downlink use propagation paths of different frequencies. Therefore, array spaces for the uplink and the downlink differ from each other, so it is necessary that control is executed on the basis of a feedback signal from a terminal station or a propagation path for the downlink is predicted. Although the propagation paths change slightly between the uplink and the downlink, the bearing of a path connecting the base station and the terminal station changes scarcely because the bearing of the terminal station does not change. A method of estimating an array weight from bearing estimation by using such a property is proposed.
A third generation communication system represented by a W-CDMA system is expected to carry out unbalanced one-way communication and packet communication in addition to conventional sound connection. In this system also, the AAA is regarded as an attractive, important means for increasing capacity. However, when the uplink and the downlink differing from each other in information transmission rate have different values of transmission power, directions in which null is to be made are generally different. Therefore, a weight which is effective for the uplink is no longer a suitable weight for the downlink. Weights respectively for the uplink and the downlink must individually be estimated, and array weights conforming to the densities of transmitted information must be estimated.
Accordingly, it is an object of the present invention to reduce base station transmission power and to increase the number of simultaneously connected terminal stations when an AAA is applied to the CDMA system to improve the downlink as well as the uplink.
The object is achieved by a cellular radio communication base station of a code-division multiple access system (CDMA system) comprising: a plurality of antennas distributed over a spatial domain; a despreading means which prepares, when a specific mobile station B is noticed, a despreading code sequence in complex conjugate relation with a spread code sequence used by the mobile station B for transmission, multiplies signals received by the antennas by the despreading code sequence at an appropriate phase difference corresponding to a propagation delay of a signal from the mobile station B to the base station, and calculates the sum total; a spatial correlation matrix estimating means that calculates the correlation between the outputs of the plurality of antennas at a code phase corresponding to a specific propagation path for a code sequence corresponding to the specific mobile station in the despreading means and stores the calculated data about the correlation; a downlink modulation means that modulates information transmitted from the base station through the downlink to the mobile station; a downlink transmission output control means that determines a transmission output for the downlink; a downlink transmission power weighting means that adjusts the output of the downlink modulating means according to an instruction given by the downlink transmission output control means; a downlink beam forming means that weights the output of the downlink modulating means so that signals of an appropriate complex envelope are radiated through the plurality of antennas; and a downlink array weight control means that calculates subspaces for mobile stations and paths on the basis of the output of the spatial correlation matrix estimating means, normalizes the subspaces by a maximum diagonal factor among those of the subspaces for all the downlinks excluding the mobile station B, weights normalized values by the downlink transmission output provided by the downlink transmission output control means, adds up the weighted normalized values to determine a subspace for an interference signal, and determines a weight of the downlink beam forming means for the mobile station B on the basis of an array response vector obtained from a correlation matrix relating to the mobile station B, and the interference subspace.
Moreover, in the cellular radio communication base station, the downlink transmission output control means determines transmission power on the basis of a power control signal provided by a mobile station or downlink signal information available from the base station.